Transfer Switch Load Calculator
Calculate total connected load, transfer switch ampacity, and recommended generator size based on your electrical loads.
Load Entries
Load Description
Watts (W)
Quantity
Formulas Used
Total Connected Load (W): Σ (Watts × Quantity) for all loads
Demand Load (W): Total Load × Demand Factor
Apparent Power (VA): Demand Load (W) ÷ Power Factor
Load Current — Single Phase (A): VA ÷ V
Load Current — Three Phase (A): VA ÷ (√3 × V)
Transfer Switch Rating (A): Load Current × 1.25 (NEC 125% continuous load factor) → rounded up to next standard size
Generator Size (kW): (VA ÷ 1000) × 1.25 headroom × Power Factor → rounded up to next standard size
Assumptions & References
- NEC 210.20 & 230.42: Overcurrent protection and conductors must be rated at 125% of continuous load current.
- NEC 702.5: Optional standby system transfer equipment must be adequate for the load served.
- NEC 445.13: Generator conductors must have an ampacity of not less than 115% of the nameplate current rating.
- Demand factor accounts for the statistical likelihood that not all loads operate simultaneously (NFPA 70 Article 220).
- Power factor of 0.85 is a typical default for mixed residential/commercial loads; adjust for motor-heavy loads (0.70–0.80).
- Generator sizing includes 25% headroom above calculated demand to accommodate motor starting surge currents.
- Standard transfer switch sizes: 30, 60, 100, 150, 200, 225, 260, 320, 400, 600 A and above.
- This calculator provides estimates only. A licensed electrician must verify all transfer switch and generator installations.